This is a basic calculation of a contaminant concentration in bulk soil based on complete, instantaneous mixing. If an interval is given, an attempt is made at calculating a long term maximum concentration using the concepts layed out for example in the PPR panel opinion (EFSA 2012).
PEC_soil(rate, rate_units = "g/ha", interception = 0, mixing_depth = 5, PEC_units = "mg/kg", PEC_pw_units = "mg/L", interval = NA, n_periods = Inf, tillage_depth = 20, chent = NA, DT50 = NA, Koc = NA, Kom = Koc/1.724, t_avg = 0, scenarios = c("default", "EFSA_2015"), porewater = FALSE)
| rate | Application rate in units specified below |
|---|---|
| rate_units | Defaults to g/ha |
| interception | The fraction of the application rate that does not reach the soil |
| mixing_depth | Mixing depth in cm |
| PEC_units | Requested units for the calculated PEC. Only mg/kg currently supported |
| PEC_pw_units | Only mg/L currently supported |
| interval | Period of the deeper mixing, defaults to 365, which is a year if degradation rate units are in days |
| n_periods | Number of periods to be considered for long term PEC calculations |
| tillage_depth | Periodic (see interval) deeper mixing in cm |
| chent | An optional chent object holding substance specific information. Can also be a name for the substance as a character string |
| DT50 | If specified, overrides soil DT50 endpoints from a chent object If DT50 is not specified here and not available from the chent object, zero degradation is assumed |
| Koc | If specified, overrides Koc endpoints from a chent object |
| Kom | Calculated from Koc by default, but can explicitly be specified as Kom here |
| t_avg | Averaging times for time weighted average concentrations |
| scenarios | If this is 'default', the DT50 will be used without correction and soil properties as specified in the REACH guidance (R.16, Table R.16-9) are used for porewater PEC calculations. If this is "EFSA_2015", the DT50 is taken to be a modelling half-life at 20°C and pF2 (for when 'chents' is specified, the DegT50 with destination 'PECgw' will be used), and corrected using an Arrhenius activation energy of 65.4 kJ/mol. Also model and scenario adjustment factors from the EFSA guidance are used. |
| porewater | Should equilibrium porewater concentrations be estimated based on Kom and the organic carbon fraction of the soil instead of total soil concentrations? Based on equation (7) given in the PPR panel opinion (EFSA 2012, p. 24) and the scenarios specified in the EFSA guidance (2015, p. 13). |
The predicted concentration in soil
This assumes that the complete load to soil during the time specified by 'interval' (typically 365 days) is dosed at once. As in the PPR panel opinion cited below (PPR panel 2012), only temperature correction using the Arrhenius equation is performed.
Total soil and porewater PEC values for the scenarios as defined in the EFSA guidance (2015, p. 13) can easily be calculated.
If temperature information is available in the selected scenarios, as e.g. in the EFSA scenarios, the DT50 for groundwater modelling (destination 'PECgw') is taken from the chent object, otherwise the DT50 with destination 'PECsoil'.
EFSA Panel on Plant Protection Products and their Residues (2012) Scientific Opinion on the science behind the guidance for scenario selection and scenario parameterisation for predicting environmental concentrations of plant protection products in soil. EFSA Journal 10(2) 2562, doi:10.2903/j.efsa.2012.2562
EFSA (European Food Safety Authority) (2015) EFSA guidance document for predicting environmental concentrations of active substances of plant protection products and transformation products of these active substances in soil. EFSA Journal 13(4) 4093 doi:10.2903/j.efsa.2015.4093
PEC_soil(100, interception = 0.25)#> scenario #> t_avg default #> 0 0.1# This is example 1 starting at p. 79 of the EFSA guidance (2015) PEC_soil(1000, interval = 365, DT50 = 250, t_avg = c(0, 21), scenarios = "EFSA_2015")#> scenario #> t_avg CTN CTC CTS #> 0 21.96827 11.53750 9.145259 #> 21 21.78517 11.40701 9.017370PEC_soil(1000, interval = 365, DT50 = 250, t_av = c(0, 21), Kom = 1000, scenarios = "EFSA_2015", porewater = TRUE)#> scenario #> t_avg CLN CLC CLS #> 0 0.7589401 0.6674322 0.9147861 #> 21 0.7506036 0.6590345 0.8987279# The following is from example 4 starting at p. 85 of the EFSA guidance (2015) # Metabolite M2 # Calculate total and porewater soil concentrations for tier 1 scenarios # Relative molar mass is 100/300, formation fraction is 0.7 * 1 results_pfm <- PEC_soil(100/300 * 0.7 * 1 * 1000, interval = 365, DT50 = 250, t_avg = c(0, 21), scenarios = "EFSA_2015") results_pfm_pw <- PEC_soil(100/300 * 0.7 * 1000, interval = 365, DT50 = 250, t_av = c(0, 21), Kom = 100, scenarios = "EFSA_2015", porewater = TRUE)